The present invention relates to a novel and improved method for the preparation of .alpha.-tetralone or, in particular, to a method for the preparation of .alpha.-tetralone by the liquid phase oxidation of tetralin with oxygen in the presence of a metal catalyst.
In the conventional procedure for the synthetic preparation, .alpha.-naphthol is prepared by the sulfonation of naphthalene with sulfuric acid followed by the alkali fusion of the sulfonation product. This procedure, however, suffers from the disadvantages of the low yield of the objective .alpha.-naphthol due to the inevitable formation of the isomeric by-product .beta.-naphthol, lower working efficiency and large volumes of noxious waste water. Therefore it is a recent trend that more and more of .alpha.-naphthol is produced by the gas-phase precious metal-catalyzed dehydrogenation of .alpha.-tetralone which is a liquid phase oxidation product of tetralin.
It is well known that the oxidation reaction of tetralin with gaseous oxygen with or without a solvent in the absence of any catalyst or in the presence of metal ions as the catalyst gives in high yields .alpha.-tetralyl hydroperoxide, .alpha.-tetralol and .alpha.-tetralone which are the selective oxidation products of tetralin oxidized at the .alpha.-position only. Two routes are proposed for the preparation of .alpha.-tetralone along the above described line of the oxidation reaction of tetralin. Firstly, highly selective oxidation of tetralin gives .alpha.-tetralyl hydroperoxide which is subsequently converted into .alpha.-tetralone by use of heavy metal ions as the catalyst (see Japanese Patent Disclosure SHO 52-10248). Secondly, .alpha.-tetralone is directly synthesized from tetralin by use of the binary catalyst composed of a soluble chromium or cobalt salt and an organic base such as amines and pyridine derivatives (see U.S. Pat. No. 3,404,183 and Japanese Patent Disclosures SHO 49-135958, 50-112347 and 51-48643).
The former method is defective because the method involves the selective synthesis of highly explosive and dangerous .alpha.-tetralyl hydroperoxide taking a long reaction time for the reaction which must be carried out at low temperatures to avoid the danger of explosion with low working efficiency. In addition, the high selectivity for the oxidative preparation of .alpha.-tetralyl hydropreoxide is obtained only when the reaction is terminated at an early stage where the conversion of tetralin is relatively low leading to increased production cost if not to mention the economical disadvantage inherent to the two-step reaction of the process.
The latter method is disadvantageous by the low selectivity for .alpha.-tetralone, especially, when a cobalt salt is used as the catalyst, with the formation of large amount of .alpha.-tetralol with relative uselessness as an industrial chemical. What is worse, the boiling point of .alpha.-tetralol is very close to that of .alpha.-tetralone. Consequently the feed material of .alpha.-tetralone in the dehydrogenation process for the preparation of .alpha.-naphthol necessarily contains considerable amount of .alpha.-tetralol as an impurity which is dehydrated into 1,2-dihydronaphthalene which is further dehydrogenated into naphthalene. Thus the preparation of .alpha.-naphthol starting with an intermediate containing large amount of .alpha.-tetralol is very disadvantageous in the process efficiency.
The latter method by use of a chromium salt as the catalyst is also disadvantageous though somewhat differently. Namely, the method is very excellent in so far as only the selectivity with respect to the relative yield of .alpha.-tetralone to .alpha.-tetralol concerns because the yield of .alpha.-tetralone is 10 to 50 times higher than the yield of .alpha.-tetralol but the method is not practical in the standpoints of economy and working efficiency due to the necessity of large amount of expensive organic bases of pyridine derivatives as well as to the very low reaction velocity and the low conversion of tetralin. Moreover, the inventors have established experimentally that large amount of .alpha.-tetralyl hydroperoxide is always formed as the by-product in the latter method in which tetralin is oxidized with gaseous oxygen in liquid phase without solvents in the presence of a pyridine base even by use of a sufficient amount of the soluble chromium salt or cobalt salt. Although the examples in the above recited Japanese Patent Disclosures are silent on the formation of the hydroperoxide, it may be a fair assumption that this method is not recommendable as an industrial procedure for the preparation of .alpha.-tetralone because of the formation of so large amount of the hydroperoxide which might be formed also in the examples of the prior art disclosures.